1. Technical Field
The present invention generally relates to a semiconductor memory device performing a read operation and an operating method thereof.
2. Related Art
FIG. 1 is a graph for describing a change in distribution of threshold voltages of memory cells by cycling and retention effects.
In general, in order to store data in memory cells of a semiconductor memory device, a threshold voltage of the memory cell is increased by performing a program operation. When a high voltage is applied to a control gate in the program operation, electrons flow in a floating gate by FN tunneling to increase the threshold voltage of the memory cell. However, according to the repeat of a program/erase operation in the memory cell, the electrons which have flowed in the floating gate are trapped in a tunnel oxidation layer, and the like. Accordingly, it is known that as the number of program/erase cycling (EW cycling) increases, program speeds of the memory cells increase.
Referring to FIG. 1, it can be seen that as the number of program/erase cycling increases, a right side of an Nth distribution becomes gradually widened.
In the meantime, stored data needs to be maintained for a long time (for example, 10 years or more) in a semiconductor memory, especially, a NAND flash memory even in a state where a power is turned off, which is referred to as reliability. According to high-integration of a flash memory, a size of the memory cell is decreased and an interval of each distribution is very narrow. When the distribution is narrow, it is necessary to sensitively adjust the number of electrons stored in the floating gate, and loss of the electrons according to a time needs to be suppressed for reliability. However, as the size of the cell is decreased and the number of electrons stored in one cell is decreased, loss of one electron exerts large influence on the distribution. The electron exhibits negative charge, so that the loss of the electron moves the distribution in a left side.
Referring to FIG. 1, it can be seen that as a time increases, a left side of an N+1th distribution is gradually widened by the retention effect.
Accordingly, there is a problem in that a left cell of a reference distribution (N+1) crosses a right cell of a directly lower distribution (N) of the reference distribution in the distribution. In order to prevent an error generated in a read operation, it is necessary to change a read voltage.